The slit lamp biomicroscope, or slit lamp as it is commonly referred to, is a versatile instrument used for examining the eye and ocular adnexae. It consists of a biomicroscope, an illumination source, and a mechanical supporting system which facilitates the positioning of the illumination source at various angles to the eye in order to achieve different kinds of examinations. The biomicroscope is an optical device which presents an enlarged image of the patient's eye to the observer and may be either a simple or compound biomicroscope with the latter having the advantages of additional magnification and less aberration than in a single lens system. Most typical prior art compound biomicroscopes present a real and inverted image, which can be corrected using a prism correction system. All slit lamp biomicroscopes achieve stereopsis by having a conversion angle between the separated oculars. However, there are many optical systems used to change magnification. These include rotating objectives, those that operate based on a Galilean telescope principle and those that function with zoom optics.
In examining the structures of the anterior segment of the eye, there are four different techniques that are presently employed with the prior art slit lamp biomicroscope. The first and most common of these is that of direct illumination. This technique involves focusing a beam of light through relatively transparent media and observing the scatter of light against a dark background. There are several forms of scatter illumination which are helpful in selected aspects of the ophthalmic examination. Thus, it is useful for this type of examination to observe scatter illumination in various parts of the eye.
A second technique of slit lamp examination is retro-illumination. With this method, light is reflected off of a diffusing surface to illuminate a more anterior structure in the eye. Thus, light may be bounced off the iris to illuminate the cornea or be reflected off of the fundus to examine the iris or lens. The image from this technique, as well as the technique of direct illumination, is degraded by reflections and by scatter from objects anterior from or posterior to the plane of interest.
A third technique in eye examination is sclerotic scatter. In this method, the incident slit light beam is oriented in an oblique fashion to the eye so that light falls on the limbal area of the cornea. As the angle of incidence is greater than the critical angle, light is reflected internally along the cornea similar to light traveling in fiber optic tubing. Additionally, some retro-illumination also occurs due to light scattered onto the iris or other posterior structures. While this technique is intended to visualize scars or opacities within the corneal stroma, the intense amount of scatter experienced reduces the resolution of the image and, hence, the value of this technique.
A fourth technique for anterior segment examination is that of specular reflection. This method utilizes the difference in index of refraction between the cornea and aqueous humor or the cornea-tear film and air, allowing visualization of either the corneal endothelium, or epithelium respectively. Occasionally, the anterior and posterior surface of the lens and the zones of discontinuity within the lens can be appreciated. Specular reflection is observed only monocularly, because the illumination beam blocks one of the observation paths. The image created with this technique of examination is degraded due to reflections from other portions of the corneal endothelium or epithelium or other ocular structures. In all four of these techniques of examination, the ocular images are also limited by the available contrast between the subject of interest and surrounding tissue. In addition, the closer the index of refraction is between the two interfaces the less the specular reflection will be, thereby reducing the amount of returning light forming the image to be observed.
While there are accessory devices available in the prior art designed to improve the aforementioned techniques of eye examination with the slit lamp biomicroscope, light reflection and scatter degrade those techniques utilizing direct illumination while limited tissue contrast and low levels of returning light limit the resolution of those techniques utilizing indirect illumination. Also, there is at least one tandem scanning confocal biomicroscope in the prior art which utilizes separate apertures and light paths to achieve some improvement over the typical slit lamp. However, difficulties of complexity and alignment are expected to limit its usefulness, as with tandem scanning confocal microscopes.
To solve these and other problems with the prior art slit lamp biomicroscope, the inventors herein have succeeded in designing and developing a single aperture confocal scanning biomicroscope, as well as two variations of a kit for converting existing slit lamp biomicroscopes into single aperture confocal scanning biomicroscopes. The many advantages of single aperture confocal scanning microscopy over dual aperture confocal microscopy are explained in the parent patents. Confocal microscopy includes the technique of illuminating only a small portion of the specimen at a time, and masking the returning (reflected or fluorescence) light to view only that same small portion to minimize the effects of scattered and out of focus light from surrounding portions of the specimen. The entire specimen is viewed by scanning it in small increments and coalescing these increments either in real time or with a video camera and image processor or the like. Other advantages of single aperture confocal scanning microscopy are to be found in the parent patents referred to, supra. With this invention, the many advantages of single aperture confocal scanning microscopy are brought to the instrument used to examine the eye and thereby minimize the major problems with the prior art slit lamp.
In one of the preferred embodiments, a significant part of the invention includes an improved illumination system. In the improved illumination system, a set of three cylindrical lenses is used to converge the light along the width of the masking aperture and an additional cylindrical lens positioned orthogonally to the first set is used to independently converge the light along the length of the masking aperture. Incorporated in the illumination system is a single assembly comprised of a cylindrical lens, masking aperture, and beam splitter which is oscillated as an integral unit which provides major advantages over other single aperture confocal scanning optical devices. Essentially, the illumination system includes a light filament and a curved reflector for concentrating a large portion of the light emanating from the filament into a pair of lenses with an aperture positioned therebetween in order to collimate the light into an incident light field. The collimating aperture is preferably a rectangular slit with cylindrical lenses, although a pin hole aperture and spherical lenses can be used. With a rectangular slit, the generally rectangular filament should be aligned with its long axis parallel to the slit's long axis. This lens pair and aperture thus aid in producing an incident light field of highly collimated light along the width of the masking aperture utilizing a large portion of the incident light produced by the filament. A cylindrical lens between the lens pair and aperture assembly focuses the incident light field along the length of the masking aperture into a field plane thereat.
The assembly mentioned above (aperture assembly), including a lens, a masking aperture, and a beam splitter, is positioned within the light field beyond the last mentioned cylindrical lens and further focuses the incident light field along the width of the masking aperture into an aperture size beam at the masking aperture. Additionally, a pivotally mounted variable V-shaped flap may be used to block a variable central portion of the light field so as to create an aperture size incident light beam comprised essentially of the edge portions of the light field. By doing so, only that light entering the lens at a relatively oblique angle is utilized which can optimally minimize the volume of intersection between the incident light and the return light path. Not only does this achieve dark field illumination of the specimen but it also minimizes scattering within all planes in the return light path with the exception of the specimen plane. This latter feature which is achieved with this "peripheral illumination" helps to maximize optical sectioning.
The particular lens used in the aperture assembly is a cylindrical lens which focuses the light along its width through the masking slit aperture. By matching the lens with the masking aperture, the aperture size beam may actually be focused within the contour of the masking aperture such that there is no backscatter from the aperture and virtually all of the incident beam passes therethrough. This further concentrates the incident light beam and provides a marked improvement to contrast as backscatter from the aperture is eliminated as a component of the light returning to the viewer. The pivotally mounted flap or peripheral illuminator may be included as part of the assembly which is oscillated within the conjugate field plane, if desired. All of the foregoing including the incident light source, lens pair and collimating aperture, cylindrical lens focusing along the length of the masking aperture, pivotally mounted flap, and aperture assembly of lens, masking aperture, and beam splitter, all comprise the illumination system which may be used to replace the slit lamp illumination system in the prior art.
Another pair of lenses then focuses the incident light beam at the specimen plane. Light reflected from the specimen is similarly focused by the same lens pair at the masking aperture and is masked thereby. Thus, light returning from the specimen is masked by the masking aperture in the preferred embodiment, but not the light illuminating the specimen. The returning light passes through the beam splitter to a biomicroscope objective as is well known in the art and the observer is thus presented with a confocal image of the eye which is illuminated either with direct illumination or with peripheral illumination. As is well known in the art, a pair of erector prisms may be used anywhere in the return light to re-orient the image in the upright direction, and hence are not shown.
The single aperture confocal scanning biomicroscope of this invention may be provided as a complete device, or the illumination system and lens pair may be provided as a retrofit kit to adapt existing slit lamp biomicroscopes for use as a single aperture confocal scanning biomicroscope.
In still another alternative, the lens used as part of the aperture assembly may be a double compound cylindrical lens; or may be a pair of cylindrical lenses bonded together, or may be a spherical lens in the event the cylindrical lens focusing along the length of the masking aperture is eliminated; or the lens may focus the light only along its width for some applications.
A somewhat simpler embodiment is also disclosed herein comprised of an aperture, an aperture oscillator, and a pair of lenses, with an alternative of a third lens or prism assembly for reinverting the image. In this embodiment, the original illumination system of a prior art slit lamp biomicroscope is utilized except that the masking aperture is placed at the original specimen plane, and the pair of lenses are used to refocus the incident light at a new specimen plane. Means are provided for moving the masking aperture within the conjugate field plane in which it is placed. In this embodiment, the masking aperture may have a multitude of openings through which the incident light may pass to illuminate the specimen. For example, a rotating Nipkow disc or an oscillating multi-slit aperture could be used. The third lens or erector prisms may be disposed in the returning light path to reinvert the image for viewing by the observer. As this embodiment utilizes the existing illumination system of the slit lamp and thus does not provide peripheral illumination, its performance is not nearly as dramatically improved as with the first embodiment described above. Additionally, in this embodiment, the mirror of the illumination system blocks a portion of the return light from the image causing resolution to suffer. Replacing the mirror with a beam splitter would prevent this blockage, but this is generally inconvenient in most biomicroscopes of prior art design.
While the principal advantages and features of the present invention have been explained above, a greater understanding of the invention may be attained by referring to the drawings and description of the preferred embodiment which follow.